Semiconductor light-emitting diodes (LEDs) are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness LEDs capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. In general, the III-nitride device layers in an LED must be epitaxial in order for the LED to function at a useful efficiency. III-nitride devices are grown by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The substrate on which a crystal is grown greatly influences the epitaxial growth mechanism and quality of the crystal. In order to grow III-nitride crystal layers of sufficient quality for use in an LED, the crystal lattice parameters of the substrate must be identical to or closely match the crystal lattice parameters of the epitaxial layers. Typically, III-nitride LEDs are grown on sapphire, SiC, or GaN substrates. Both sapphire and SiC are mismatched from the GaN, sapphire by 15% and SiC by 3.5%. III-nitride LEDs structures are often grown on sapphire substrates due to sapphire""s high temperature stability and relative ease of production.
The use of a sapphire substrate may lead to poor extraction efficiency due to the large different in index of refraction at the interface between the semiconductor layers and the substrate. When light is incident on an interface between two materials, the difference in index of refraction determines how much light is reflected at that interface, and how much light is transmitted through it. The larger the difference in index of refraction, the more light is reflected. The refractive index of sapphire (1.8) is low compared to the refractive index of the III-nitride device layers (2.4) grown on the sapphire. Thus, a large portion of the light generated in the III-nitride device layers is reflected when it reaches the interface between the semiconductor layers and a sapphire substrate. The reflected light must scatter and make many passes through the device before it is extracted. These many passes result in significant attenuation of the light due to optical losses at contacts, free carrier absorption, and interband absorption within any of the III-nitride device layers.
The index of refraction of SiC (2.7) more closely matches the index of refraction of the III-nitride device layers. However, as described above, sapphire and SiC have a lattice mismatch from GaN. As a result of the lattice mismatch, buffer or nucleation layers which are optimized for lattice matching and coefficient of thermal expansion matching between the substrate and the III-nitride device layers are typically grown on the substrate before the III-nitride device layers. FIG. 1 shows an example of buffer layers used on SiC substrates, described in U.S. Pat. No. 5,393,993. A three layer buffering structure comprising layers 26, 22, and 23 is formed between the SiC substrate 25 and the epitaxial GaN layer 24. The layer immediately adjacent to the SiC substrate is AlN. This AlN buffer layer, which has an index of refraction of about 2.0, reduces most of the light extraction benefit that may be derived from the use of SiC.
In accordance with the invention, a light emitting device includes a nucleation layer containing aluminum. The thickness and aluminum composition of the nucleation layer are selected such that 90% or more of light from the device layers incident on the nucleation layer is extracted into the substrate. In some embodiments, the nucleation layer is AlGaN with a thickness between about 600 and about 2000 angstroms and an aluminum composition between about 2% and about 8%. In some embodiments, the nucleation layer is formed over a surface of a wurtzite substrate that is miscut from the c-plane of the substrate. In such embodiments, the substrate may be slightly miscut, for example by between 0xc2x0 and 5xc2x0 from the c-plane, or the substrate may be largely miscut, for example by between 30xc2x0 and 50xc2x0, 80xc2x0 and 100xc2x0, or 130xc2x0 and 150xc2x0 from the c-plane. In some embodiments, the nucleation layer is formed at high temperature, for example between 900xc2x0 and 1200xc2x0 C. In some embodiments, the nucleation layer is doped with Si to a concentration between about 3e18 cmxe2x88x923 and about 5e19 cmxe2x88x923.